1. Field of the Invention
The present invention relates to a semiconductor memory device having a bipolar transistor and, more specifically, to a semiconductor memory device having a bipolar transistor which can be manufactured at low cost without impeding high speed of operation.
2. Description of the Background Art
A bipolar transistor has been conventionally used in various devices. FIGS. 10 and 11 show a bipolar transistor which has been generally used. FIG. 10 is a plan view of a conventional bipolar transistor. FIG. 11 is a cross section taken along the line XI--XI of FIG. 10. Structure and method of manufacturing the bipolar transistor having such structure are described in detail in Paul R. Gray and Robert G. Meyer, Analysis and Design of Analog Integrated Circuits, Second Edition, Chapters 2.3-2.4, for example.
The structure of the conventional bipolar transistor will be described with reference to FIGS. 10 and 11. Referring to FIGS. 10 and 11, an n type buried diffusion layer 2 having an n type impurity at a high concentration is formed at the main surface of a p type semiconductor substrate 1. On the n type buried diffusion layer 2, an n type epitaxial layer 3 is formed. The n type epitaxial layer 3 is formed by epitaxial growth, and includes an n type impurity at low concentration. A p type base region 4 is formed on the surface of n type epitaxial layer. An n type emitter region 6 is formed on the surface of p type base region 4. An insulating layer 7 is formed to cover the surface of n type epitaxial layer 3, and openings 8a, 8b and 8c are formed at prescribed positions of insulating layer 7. A collector electrode 9a is formed in opening 8a, a base electrode 9b is formed in opening 8b, and an emitter electrode 9c is formed in opening 8c. Collector electrode 9a, base electrode 9b and emitter electrode 9c are preferably formed of a metal including Al. On the surface of n type epitaxial layer 3 which is in contact with collector electrode 9a, an n.sup.+ diffusion layer 5 having an n type impurity at high concentration is formed.
Concentration profile of the impurity in the depth direction of the semiconductor substrate in the conventional bipolar transistor having such structure as described above is as shown in FIG. 12. Referring to FIG. 12, below n type emitter region 6, p type base region 4 is formed, and below the p type base region 4, n type epitaxial layer 3 including the n type impurity of about 1.times.10.sup.15 cm.sup.-3 is formed. Below n type epitaxial 3, n type buried diffusion layer 2 including n type impurity of high concentration is formed. In the conventional bipolar transistor of the type as described above, Collector resistance is reduced as it has n type buried diffusion layer 2, and breakdown voltage between the collector/emitter can be maintained. Therefore, a bipolar transistor with high performance can be obtained. However, in the bipolar transistor of the type described above, epitaxial growth of the substrate is necessary. This results in significant increase in manufacturing cost.
Another example of the conventional bipolar transistor is of the type shown in FIG. 13. FIG. 13 is a cross section showing another type of the conventional bipolar transistor. The bipolar transistor of the type shown in FIG. 13 includes an n type collector region 3a, a p type base region 4 and an n type emitter region 6. The bipolar transistor of this type is formed by diffusing impurity from the surface of p type semiconductor substrate 1. Therefore, as compared with the bipolar transistor shown in FIGS. 10 and 11, manufacturing cost can be reduced.
However, the bipolar transistor of the type shown in FIG. 13 has the following disadvantages. The disadvantages will be described with reference to FIG. 14. FIG. 14 shows concentration profile of the impurity in the depth direction of p type semiconductor substrate 1 in the bipolar transistor of the type shown in FIG. 13.
Referring to FIG. 14, the n type impurity concentration included in n type collector region 3a is low at a portion deep in the substrate. This results in increased collector resistance. As the collector resistance increases, there is greater voltage drop when current flows in the collector region. Therefore, it is possible that the base/collector is strongly biased in the forward direction. If the forward direction bias is 400 mV or more, the bipolar transistor is saturated, and the speed of operation is significantly reduced.
Further, the bipolar transistor of the type shown in FIG. 13 also has the disadvantage that the breakdown voltage between collector/emitter is reduced.